1. Field of the Invention
This invention relates generally to distribution power systems including clamp devices for preventing arc damage to insulated distribution conductors, and especially to such systems protected by one or more fuses.
2. Description of the Prior Art
Electric power lines operating between 4 kV and 34 kV line-to-line are usually classified as distribution lines. In the United States, both bare and insulated distribution line conductors are used. Bare overhead distribution conductors are insulated from ground with stand-off or string insulators connected to support poles. Insulated conductors, which comprise a significant percentage of installed power distribution lines, have an insulation cover to reduce hazards to life and property near the line. Insulated conductors also provide certain other advantages over bare conductor circuits. For example, momentary tree contact is less likely to fault a covered conductor than a bare conductor. Momentary line-to-line contact caused by wind deflection can fault a bare conductor circuit; a covered or insulated conductor circuit would be unaffected by such contact.
There is, however, the hazardous phenomenon of conductor burndown associated primarily with insulated distribution conductors. Lightning can strike an overhead distribution conductor anywhere along its length, often creating a fault current path by arcing to another conductor at a weak point (commonly known as lightning-induced flashover). The most probable arcing occurs with common vertical lines having current flowing between the top phase and the neutral conductor. Flashover can also occur from the top conductor to another phase conductor, sometimes involving all lines and the neutral.
After formation, the arc moves along the conductor away from the power source, due to electromagnetic forces. If the arc's travel is in some way restricted, the arc energy is concentrated on a small portion of the conductor. The concentrated energy causes the conductor to lose tensile strength, and it can eventually break and fall to the ground. This is the burndown phenomenon. The restriction can be caused by a puncture in or stripping away of the insulation covering of a distribution conductor, providing a highly probable lightning flashover path.
The amount of conductor damage at the point of arcing varies with the conductor temperature produced by the fault current heat which depends, in turn, on the magnitude and duration of the fault current. The magnitude of the fault current caused by the arc is a function of the line voltage, the circuit impedance, and other system parameters. Other conditions such as arc bending winds and humidity also affect the fault current magnitude. Fault current duration depends on the speed of the protective relays in opening the faulted circuit. Often, a single arcing event is sufficient to melt enough conductor to cause it to fall to the ground. In other cases, it may require two or three arc faults at the same point over a period of twenty or thirty years to produce burndown. Also, when the arc damage has reduced the current carrying capacity of the line, normal load current heating can sometimes cause burndown.
When an overhead conductor in a multi-grounded neutral distribution system breaks and falls to the ground without simultaneously contacting the multi-grounded neutral conductor, there is a significant probability of it coming to rest on a high impedance surface such as concrete, asphalt, or dry earth. The resulting fault current may be insufficient to cause operation of the protective relaying equipment. The problem is further aggravated by use of covered phase conductors that may increase the fault impedance and further reduce the fault current magnitude. In addition to interrupting customer service, the undetected live wire is a threat to public safety and a fire hazard.
Clearly, to improve on the known reliability and safety advantages of insulated distribution lines, conductor arcing damage must be avoided where the insulation is stripped off for interconnection or support. Thus, covered conductors must be protected from burndown.
Two pending patent applications, U.S. patent application Ser. No. 248,788, filed Mar. 30, 1981 and U.S. patent application Ser. No. 248,789, filed Mar. 30, 1981, both assigned to the assignee of the present invention, teach another embodiment of the metallic clamp claimed in the present invention. Removal of the insulating cover from a distribution line conductor exposes a bare conductor segment and forms two insulating cover end faces. The prior art clamp is securely fastened to the bare conductor segment, adjacent to the end face that is farthest from the power source (in a radial distribution circuit) to provide additional heat sink mass on that portion of the bare conductor segment where the fault-current arc dwells. The bare conductor passes continuously through the clamp. Since the clamp, rather than the bare conductor, absorbs the arc energy, conductor burndown is prevented.
The prior art clamp includes an enlarged flange portion placed in proximate relation to the end face to provide additional metallic mass and to bar arc travel from the clamp to the insulating cover. The prior art clamp also includes splatter shields located on that face of the clamp opposite the enlarged flange portion for preventing molten metal, produced when the clamp absorbs the arc energy, from splattering onto the support insulator(s). Longitudinal slots of optimum width are specifically designed into the clamp to prevent the upper and lower clamp members from welding together when the arc energy is absorbed, while preventing arc travel along that portion of the conductor exposed by the slots. Lastly, in the prior art clamp, the bolt holding the clamp members together is embedded in the clamp over nearly the bolt's entire length. This design feature prevents the arc from jumping from the clamp to the bolt tip, and welding the tip to the clamp.
Distribution lines are protected from faults by protective relays operating interrupt devices, or fuses; the latter are used on approximately 60-75% of all overhead distribution lines. On distribution lines using only protective relays operating interrupt devices, the arc can last for 6-12 cycles before the protective relay opens the interrupt device to extinguish the arc. The arc's duration requires that the prior art clamp mass be proportional to the arc energy. Theoretical and empirical studies were used to determine the proper mass for the prior art clamp. Before it is extinguished, the arc causes the entire mass of the prior art clamp to be heated. Consideration must therefore be given to heat transfer and distribution in the prior art clamp and it must be contoured accordingly. On fused lines, however, the arc is extinguished after approximately 1/2 cycle by opening of the fuse. More arc energy must therefore be dissipated by clamps used on lines protected by protective relays in conjunction with interrupt devices than those protected by fuses.
The prior art clamp, designed for distribution lines protected by protective relays, has substantial mass, and cost, due to the large amount of arc energy it must absorb. The geometry is also complicated by the large heat-sinking capabilities required and heat transfer and distribution throughout the clamp. Of course, this complicated geometry adds to fabrication cost. Recognition of the shorter arc-time associated with fuse-protected distribution lines allows several improvements to be made in the prior art clamp without sacrificing its protection features. The most significant improvement involves mass reduction. Empirical studies have indicated that for fused lines, the clamp mass necessary for practical structural characteristics is sufficient to provide adequate heat-sink mass for the largest available fuse, i.e., the fuse which would take the longest time to extinguish the arc. That is, the amount of mass of the present clamp necessary to give it the required structural features is sufficient to absorb the arc energy on any fused distribution line because the fuse limits the arc's duration. The clamp of the present invention, intended for use only on fuse-protected lines, has approximately 1/3 the mass of the prior art clamp. The prior art clamp has a mass of 294 g, and the present clamp has a mass of 92.1 g. This mass reduction translates into a 1/3 cost reduction. These and other advantages of the present invention are discussed below.